If contacting customers and suppliers is an important part of your business, you will want to set your work schedule according to their availability. When you start out in business, you may want to hold to the traditional business hours of your locale, or to the hours of your type of business. After you have been in operation for a while, you may get a feel for whether you could better serve customers and get an advantage over your competitors by adjusting your work schedule.
You have much more freedom to choose your business hours if you don't speak
directly with customers or suppliers. If you work best in the wee hours of the
morning, do it! Being your own boss was probably one of the reasons that you
went into business for yourself. You should take advantage of this freedom when
you can — you'll feel better about what you're doing and be more productive.